The present invention relates to hydraulic pumps. More particularly, the present invention pertains to a compact, smart material element driven pump capable of power output significantly beyond that available in the prior art.
Smart materials are materials that respond with a change in shape and/or change in material characteristic upon application of externally applied driving forces. Shape changing smart material elements such as those made of piezoelectric, magnetostrictive, electrostrictive or similar materials are conceptually capable of serving as the force and motion inducing mechanism in actuators and fluid pumps. These smart materials convert energy stored in electric or magnetic fields to mechanical forces and motions. Very high force can be exerted by a small actuator and controlled very rapidly, in the 1 kHz frequency range and above. However, one significant drawback of smart material actuators has been the very small displacements afforded. Devices utilizing a single stroke of the smart material element for actuation find limited use. Various displacement amplification means have been employed such as levers and various nesting schemes. Displacement amplifications of 2 to 10 times may be achieved but the overall displacement remains very small and force output is greatly reduced. As a result, the useful mechanical work output from the device is insufficient for many applications.
Recent work in the smart materials community has described the concept of a smart material actuator driving a pump in a step and repeat type fashion, that is where a small quantity of fluid is drawn into a chamber and expelled at higher pressure in a rapidly repeated fashion so to supply the necessary large volume of fluid to a large stroke actuator. Such a device has been described by Mauck and Lynch in a paper presented at the SPIE Conference on Smart Structures and Integrated Systems in March 1999. The approach therein utilized a piezoelectric element to drive a piston in a sleeve with the piston sealed with an o-ring. Relatively low pump drive rates and flows were achieved, however.
A magnetostrictive water pump was developed for NASA by Gerver, et al., and is reported in a paper presented at the SPIE Conference on Smart Structures and Integrated Systems in March 1998. O-ring sealing of a sliding piston was employed. A pump with power efficiency of only 1-2% was achieved and at very low fluid working pressure (5 psi).
A pump developed by Sirohi and Chopra of the University of Maryland utilizes a piston displacing a diaphragm spanning across the inner diameter of a cylinder so to compress fluid within a cylindrical chamber where fluid is controlled to be expelled and drawn in through the use of ball check valves. Pumping frequency was limited to around 250 Hz due to valve dynamics.
The above mentioned work has not achieved the higher flow and large power (50-500 W) in a compact design that is necessary for stand alone actuator requirements, such as for driving the aerodynamic control surfaces of an aircraft.
The patent prior art similarly fails to achieve compact, high power performance. A major reason is that smart material driven actuator drive frequencies remain severely limited and far below the drive frequency potential of the smart material element. U.S. Pat. No. 4,983,876 discloses a piezoelectric pump assembly where a piezoelectric element drives a motion amplification lever which in turn drives a diaphragm at the end of a cylindrical chamber which has ball or poppet style check valves. The significant mass and compliance in the amplification mechanism limits the operational frequency of the device to around 35 Hz, thus substantially under utilizing the high frequency drive capability of piezoelectrics. Additionally, the cylindrical pumping chamber of this device limits the volumetric pumping efficiency.
Volumetric pumping inefficiency is common to the devices of the prior art. A magnetostrictive pump is disclosed in U.S. Pat. No. 4,795,318 where a magnetostrictive element drives an o-ring sealed piston to pump fluid across ball/spring check valves. Pumping efficiencies and pumping frequencies remain relatively limited by the compliance of the o-ring seals associated with the sliding piston and the low operational frequency associated with ball check valves. U.S. Pat. No. 5,641,270 discloses a magnetostrictive pump wherein a magnetostrictive element drives a diaphragm in a cylindrical chamber which further includes a bellows. The working fluid compressed in the chamber further compresses the bellows which pumps a second fluid. Volumetric inefficiency of the cylindrical chamber and uncontrolled deformations in the diaphragm due to diaphragm membrane and bending compliances significantly diminish the volumetric pumping efficiency of the device.
The usefulness of the devices of the prior art has been limited by their low flow and low power capability and/or lack of compactness due to low operating efficiencies. There remains a need for a self-contained, electrically driven, compact, high power hydraulic pump. Such a device would allow applications to eliminate the routing of high pressure hydraulic fluid from a central reservoir to a hydraulic actuator. The invention described herein provides for such a device.
The invention disclosed is a compact, piezoelectric or other smart material element driven pump capable of power output significantly beyond that available in the prior art. The invention achieves both a volumetric pumping efficiency and pumping frequency substantially higher than that available from smart material driven pumps of the prior art, and further, the invention combines these attributes in a compact, high power density package.
The invention combines a novel, compliance controlled pumping chamber and actuator drive construction such that the smart material element yields significantly higher volumetric pumping efficiency, i.e. the ratio of pumping chamber volume change divided by original pumping chamber volume, than that available from devices of the prior art. In the embodiments of the invention, the pumping chamber volume change and consequent pumping occurs without the sliding motion common to piston and sleeve pumps and without the effective compliance of o-ring seals or other compliant seals common to other smart material driven pumps of the prior art.
In a preferred embodiment of the invention, a diaphragm plate comprised of a stiff center pedestal is surrounded by two concentric annular flexures which provide sufficiently low bending stiffness to the diaphragm for pumping deformation. The stiff center pedestal and concentric flexure combination provide diaphragm elasticity without the unwanted and efficiency wasting compliances of devices of the prior art. Further, in a preferred embodiment, the diaphragm plate includes a shallow, conical edged recess which flattens against a face plate. A face-seal surface of a stiff material surrounds the recess and serves as an effective seal between the diaphragm plate and face plate during pumping. The effect is a broad, low profile pump chamber with negligible seal compliance. This combination of controlled diaphragm compliance, conical edge shaped pumping chamber, and stiff face-seal dynamic sealing maximizes the volumetric efficiency in each pump cycle. This in turn maximizes pressure increase and flow during pumping for a given stroke length of the smart material element. Further in the embodiment, the smart material element presses directly against the stiff center pedestal in the center of the diaphragm plate so to minimize the compliance between the smart material element and diaphragm. The extension and contraction motion of the smart material element is thereby directly imparted to the diaphragm with negligible loss of motion.
The invention combines high volumetric pumping efficiency and concomitant high pumping pressure with a pumping frequency capability substantially higher than that available in the prior art. In a preferred embodiment, reed valves of very high resonant frequency are utilized as check valves to control passively the fluid flow into and out of the pumping chamber and afford high pumping frequency. Further the absence of o-ring seal compliance or other unwanted chamber compliances upon the dynamics of the pumping chamber provides for high frequency fluid pumping.
In a further embodiment the invention provides for a self-contained actuator through the direct combination of a hydraulic actuator, smart material driven pump, and associated hydraulic valves and accumulators.